JNTU, B.Tech I Semester ELECTRO MECHANICS-I,November 2008

(Electrical & Electronic Engineering)

SET-1

1. (a) Derive expression for the forces developed in linear electromagnetic system.

(b) All energy conversion devices use magnetic field as a coupling medium rather

than electrical field. Explain Why?

2. Determine a suitable winding scheme for a 2-circuit, 4-pole winding of a D. C.

machine. The armature slots are 17 and the commutator bars are 51. Give the winding table and use retrogressive type winding.

3. (a) Explain the purpose of laminating the armature core of a d. c. machine.

(b) The diameter of a commutator ring of a lap wound d. c. motor is 25 cms. The brush width is 1.8 cm and width of mica insulation is 0.2 cm. If the speed of the motor is 200 rpm, determine the time of commutation.

4. In a 110V DC compound generator, the resistance of the armature shunt field

series field resistances are 0.06 ohm, 25 ohms and 0.04 ohm respectively. The load

consists of 200 amps each rated at 55W, 110V. Find the total emf generated and the armature current when the machine is connected in

(a) long-shunt and

(b) short shunt.

5. A 4-pole 250V DC shunt motor has lap connected 960 conductors. The flux per pole is 20mWb. Determine the torque developed by the armature and the useful torque in Nm when current drawn by the motor is 32A. The armature resistance is 0.1 and shunt field resistance is 125. The rotational losses of the machine amount to 825W. Derive the formula used.

6. (a) A 220V shunt motor takes 60A when running at 800rpm. It has an armature

resistance of 0.1. Find the speed and armature current if the magnetic flux is weakened by 20%, contact drop per brush = 1V. Total torque developed remains constant.

(b) A 220V series motor runs at 800 rpm, when taking a current of 15A. The motor has Ra = 0.3 and Rf = 0.2. Find the resistance to be connected inbseries with armature if it has to take the same current at the same voltage at 600 rpm. Assume flux is proportional to current.

7. (a) Explain the merits and demerits of a brake test on a DC motor?

(b) A 230V DC shunt motor having an armature resistance of 0.3 takes 3A excluding a field current of 1A when running on no-load. Determine its efficiency taking currents as

i. 30A;

ii. 50A and

iii. 70A.

8. (a) Explain with a neat circuit diagram, how can you find the efficiency of two

similar DC series motors of large capacity? Calculate

i. the moment of inertia of the armature in kg-m2

ii. iron loss and

iii. the mechanical lisses at the mean speed of 1015 rpm.

(b) Two identical DC machines when tested by Hopkinson’s method gave the following test results: Field currents are 2.5A and 2A, Line voltage is 220V Line current including both the field currents is 10A. Motor armature current is 73A. The armature resistance of each machine is 0.05 . Calculate the efficiency of both the machine.